Process for the distillation of decanted oils for the production of petroleum pitches

ABSTRACT

This invention describes a process for the distillation of decanted oil to improve the efficiency of the production of pitches having excellent physical and chemical properties through the development of a distillation process which basically comprises the introduction of a stage of condensation of light volatile compounds derived from distillation of the decanted oil, causing these light compounds to be recycled to the hot zone of the still and react with the raw material acting as a solvent phase which has a lower viscosity than the reaction system, within a particular temperature range and for a particular time.

FIELD OF THE INVENTION

This invention has scope for application in the processes for thedistillation of decanted oil with the aim of increasing the productionyield and quality of the pitches obtained.

BACKGROUND TO THE INVENTION

Pitches are used for the production of carbon artefacts, especially forthe production of carbon fibres having excellent mechanical properties.They also have application as binders in the industries for theproduction of anodes, aluminium and graphite electrodes for the steelindustry.

The manufacture of pitches conventionally uses coal tar as a rawmaterial.

However, environmental problems deriving from the use of coal tar, theinstability in the supply of this raw material and the great variationsin the price of this item has awoken industry's interest in alternativesources of raw material originating in the heavy residues of oilprocessing.

Conventionally the production of pitches from decanted oil involves aprocessing stage in which the raw material is distilled with removal ofthe lighter compounds, leaving as the end product a pitch which hassolid properties at ambient temperature. The mean yield from aconventional process such as that described above is approximately 20%of pitch.

RELATED ART

Document U.S. Pat. No. 4,705,618 [Maruzen Petrochemical Co., Ltd.],included here as a reference, describes a process for the continuouspreparation of a “virtually” optically isotropic and substantiallyuniform pitch intermediate for the manufacture of a pitch binder for thepreparation of carbon fibres, which comprises heating the heavy oilcontaining a quinoline-insoluble fraction of less than 5% by weightselected from a group which can be selected from tars from variousorigins having coal as a base, by-products of naphtha cracking,by-products of diesel cracking and decanted oils in a tubular heater atparticular pressures and temperatures, for a particular residence time.The flow from this heater is transferred to a distillation column whereit is distilled at a specific pressure and temperature so that the lightfractions derived from the heavy fraction are separated out as productfrom the top of this column and the heavy fraction is collected from thebottom of the column as an intermediate pitch.

Document U.S. Pat. No. 4,820,401 [Koz lizuka, Maruzen Petrochemical Co.,Ltd.], included here as a reference, discloses a process for thepreparation of a pitch with a softening point below 350° C. for themanufacture of high performance carbon fibres which comprises subjectinga heavy oil from oil or coal or a heavy component obtained from thatheavy oil by distillation to heat treatment or hydro-treatment in atubular heater under pressure for a particular residence time. In asecond stage, a monocyclic aromatic hydrocarbon solvent is added to theheat-treated material and the new insoluble component formed isrecovered essentially as a high molecular weight isotropic bituminousmaterial. In a third stage the previously obtained bituminous materialis subjected to hydrogenation treatment at a particular temperature andpressure with the addition of a solvent based on a hydrogen donor,essentially yielding an isotropic hydrogenated pitch and finally, in afourth stage, the compound from the third stage is subjected to heattreatment in a temperature range from 350° C. to 500° C. under apressure above atmospheric to convert it into a pitch binder.

Document U.S. Pat. No. 4,931,162 [Conoco Inc.], included here as areference, discloses a process for the preparation of a pitch suitablefor the production of high-quality carbon fibres which comprisesdistillation from a feed containing aromatic compounds, an aromaticdistillate free from monophase resins having an initial boiling point of390° C. at atmospheric pressure. The distillate is subjected toparticular temperatures for a time, at atmospheric pressure, so as toobtain a distillate free from mesophases, but containing a percentage ofat least 5% of mesophase resins. The free distillate is also subjectedto temperatures in the range from 370° C. to 420° C. under atmosphericpressure for a particular time in the presence of an inert gas in orderto convert the latter resins into a pitch binder.

Document U.S. Pat. No. 5,032,250 [Conoco Inc.], included here as areference, discloses a process for the preparation of a pitch binderwhich comprises combining an isotropic pitch containing mesogens with asolvent, then performing a phase separation of the mesogens with solventunder supercritical temperature and pressure conditions in such a waythat these mesogens are associated with the solvent to form a pitchbinder, which is recovered.

U.S. Pat. No. 5,259,947 [Conoco Inc.], included here as a reference,discloses a solvated mesophase pitch having a liquid-crystallinestructure comprising a vehicle of mesogens, pseudo-mesogens or a mixturethereof in which the solvated mesophase pitch is at least 40% by volumeoptically anisotropic and in which the solvated mesophase pitch melts atat least 40° C. below the mesogen component or in which the solvatedmesophase pitch contains pseudo-mesogens, where the solvated mesophasepitch melts or softens and the pseudo-mesogen component does not; wherethe solvent dissolves in the mesogen or pseudo-mesogen and gives rise toa lower melting point when it retains a substantial volume ofliquid-crystalline structure.

From what it is possible to learn from the representative examples ofthe state of the art described above, in almost all if not all of thesethere is a need to control temperature, but the temperatures aregenerally very high. Likewise it is not infrequent that the processeshave to be carried out at a pressure above atmospheric. On the otherhand it will be noted that light fractions are effectively removed fromthe process and in almost all there is the need to introduce a solventin order to obtain the pitch. Finally it will be seen that concern lieswith the physical and chemical qualities of the pitch for the productionof carbon fibres, but not with the efficiency of the process as such.

SUMMARY OF THE INVENTION

This invention relates to a process for the distillation of decanted oilin order to increase the yield of the production of pitch binders withexcellent physical and chemical qualities which makes them suitable foruse in various industrial applications.

The above aim has been accomplished through developing a distillationprocess which basically comprises the introduction of a stage of thecondensing the light and volatile compounds deriving from thedistillation of decanted oil, these light compounds being recycled tothe hot zone of the still where they react with the raw material forminga mixture which has less viscosity than the reaction system, which actsas a solvent phase. This stage is carried out at a particulartemperature and for a particular time.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a diagrammatical view of the distillation system according tothis invention.

FIG. 2 shows a graph of heating in a first distillation experiment witha cooling coil according to this invention, in which graphs of thetemperature within the still and the temperature at the top of thedistillation column are plotted.

FIG. 3 illustrates a graph of heating in a second distillationexperiment with a cooling coil according to this invention, in whichgraphs of the temperature within the still and the temperature at thetop of the distillation column are plotted.

FIG. 4 illustrates a graph of heating in a third distillation experimentwith a cooling coil according to this invention, in which graphs of thetemperature within the still and the temperature at the top of thedistillation column are plotted.

FIG. 5 shows a graph of heating in a fourth distillation experiment witha cooling coil according to this invention, in which graphs of thetemperature within the still and the temperature at the top of thedistillation column are plotted.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to a process for the distillation of decanted oilwith a view to increasing the production yield of pitches havingexcellent physical and chemical properties.

As already mentioned previously, pitches can be produced using coal tarand heavy oil residues. Heavy oil residues or residues from itsprocessing have been widely used as a primary material for obtainingpitches. However, the majority of the technical work which usespetroleum pitch in its experiments mentions the use of commercialpitches as a raw material. The production of pitches from decanted oilas raw material involves a distillation which removes lighter compoundsand makes it possible to form a heavy material, a pitch which is in thesolid state at ambient temperature. The yield of pitch obtained fromconventional distillation is of the order of 20%. A vacuum whichalthough accelerating the distillation process also results in evenlower yields than in the conventional process may be applied duringdistillation.

On conducting a first experiment in the course of bench research for thedevelopment of this invention a set-up with conventional equipment wasused, with a system for heating the column which made it possible toaccelerate the removal of light volatile compounds, together withthermal insulation to prevent the loss of heat to the exterior. Theresults are shown in Table 1 as “Run 1” below.

The abbreviations common to all the tables in this report need to beexplained, to provide a better understanding:

-   T_(int)—internal temperature of the still,-   T_(top)—temperature of the top of the distillation column,-   Time—distillation time,-   Y_(Pitch)—yield of pitch,-   SP—softening point,-   TI—compounds insoluble in toluene,-   QI—compounds insoluble in quinoline.

A second experiment was then performed in which the distillation columnwas not heated. The results are shown as Run 2 in Table 1 below.

TABLE 1 T_(int) T_(top) Time Y_(Pitch) SP TI QI Distillation max (° C.)max (° C.) (h) (%) (° C.) (%) (%) Run 1 485 454 2.5 25.6 85 6.3 0 Run 2485 427 2.7 29.4 92 9.3 0.7

It was noted that lack of heating for the column resulted in an increasein the yield of pitch, although with similar heating parameters. Theseresults led to the construction of a still (1) of larger capacity withan associated stove (11) for heating. Said still (1) was equipped with areflux system comprising a coil (2), preferably of copper, surrounding adistillation column (3). A cooling fluid, which can be selected fromwater, compressed air or any other fluid capable of exchanging heat,flows through coil (2). The still described above can be seen as adiagrammatical illustration in FIG. 1.

As already mentioned above, the process according to this invention isbased on heat treatment with reflux.

Heating of the decanted oil and reflux allows light molecules whichwould be withdrawn from the system by being volatilised to return andremain within the still. This reduces excessive condensation of theheavier molecules, purely because of the presence of these lightermolecules which have considerable aromaticity and naphthenic hydrogenswhich improve the physical and chemical properties of the pitch.

Heat treatment under reflux almost completely decomposes the paraffinfractions, bringing about dealkylation of the alkyl-aromatics, as wellas condensation of the aromatic components, especially the lighterfractions.

In this way a more uniform distribution of the components can beobtained in order to increase the yield of pitch, bringing about anincrease in toluene-insoluble (TI) compounds without excessively raisingthe softening point (SP) of the pitch.

Distillation of decanted oil with the application of reflux in this wayinhibits the volatilisation of low molecular weight components, andcauses them to condense out. These compounds, which have a low molecularweight, on the other hand form a solvent phase which has less viscositythan the reaction system. In this way these low molecular weightcomponents inhibit fast condensation of the components of greatermolecular weight, help break the chains in the latter components, anaction which delays the formation of quinoline-insoluble compounds, andalso almost entirely eliminate the need to introduce external solventcompounds into the system.

The still as constructed was used in a further four experimental runsfor which results are shown in Table 2 below, with an indication ofwhether reflux (Ref) is used or not.

TABLE 2 T_(int) T_(top) max max Time Y_(Pitch) SP TI QI Distillation (°C.) (° C.) (h) (%) (° C.) (%) (%) Ref Run 3 445 309 3.4 36.9 104 9.3 1.1None Run 4 435 293 9.3 42.0 95 11.0 1.3 water Run 5 433 263 7.8 49.3 8314.3 1.8 water/ air Run 6 432 272 9.0 53.3 86 17.8 2.4 Air

The experiments for which values are illustrated in Table 2, also knownas Runs, will be described below:

In order to carry out Run 3, 7580 g of decanted oil was placed in astill (1) prepared according to this invention, equipped with adistillation column (3) fitted with a coil (2). Distillation wasstarted, but without cooling distillation column (3), coil (2) remainingunused. Distillation continued for three hours and twenty-five minutes.The maximum temperature (T_(int)) in the still reached 445° C. Themaximum temperature at the top of the distillation column reached 309°C. With these parameters and this technique a yield of 36.9% of pitchwas obtained. FIG. 2 illustrates the heating graph for this Run 3 inwhich the change in the temperature of the still (T_(int)) is plotted onthe graph in the form of lozenges and the change in the temperature atthe top of the column (T_(top)) is plotted in the form of triangles.

In order to carry out Run 4, 7580 g of decanted oil was placed in astill (1) prepared according to this invention, equipped with adistillation column (3) fitted with a coil (2). Distillation was begunwith cooling of distillation column (3) using water as the cooling fluidin coil (2), and the temperature in the still (T_(int)) stabilised at377° C. Distillation continued with reflux for seven hours, after whichthe flow of cooling water was interrupted and the distillation processwas allowed to continue for a further two hours and twenty minutes, Themaximum temperature in the still (T_(int)) reached 435° C. The maximumtemperature at the top of the distillation column reached 293° C. Withthese parameters and this technique a yield of 42.0% of pitch wasobtained. FIG. 3 illustrates the heating graph for this Run 4 in whichthe change in the temperature of the still (T_(int)) is plotted on thegraph in the form of squares and the change in the temperature at thetop of the column (T_(top)) is plotted in the form of solid circles.

In order to carry out Run 5, 7540 g of decanted oil was placed in astill (1) prepared according to this invention, equipped with adistillation column (3) fitted with a coil (2). Distillation was begunwith cooling of distillation column (3) initially using water as thecooling fluid within coil (2), and the temperature within the still(Tint) stabilised at 376° C. Distillation continued with reflux forthree hours and thirty-eight minutes, after which the flow of coolingwater was interrupted. The distillation process was extended for afurther forty-five minutes and then compressed air was used as thecooling fluid for a further two hours. After this time cooling ofdistillation column (3) was interrupted and distillation continued for afurther one hour and ten minutes. The maximum temperature within thestill (T_(int)) reached 433° C. The maximum temperature at the top ofthe distillation column reached 263° C. With these parameters and thistechnique a yield of 49.3% of pitch was obtained. FIG. 4 shows theheating graph for this Run 5 in which the change in the temperaturewithin the still (T_(int)) is plotted on the graph in the form ofsquares and the change in the temperature at the top of the column(T_(top)) is plotted in the form of solid circles.

In order to carry out Run 6, 7480 g of decanted oil was placed in astill (1) prepared according to this invention, equipped with adistillation column (3) fitted with a coil (2). Distillation was startedwith cooling of distillation column (3) using only compressed air as thecooling fluid. Distillation continued with continuous cooling ofdistillation column (3) throughout the eight hours of the process. Thetemperature over these eight hours gradually and slowly rose until themaximum temperature within the still (T_(int)) reached 432° C. Heatingwas continued for a further hour. FIG. 5 shows the heating graph forthis Run 6 in which the change in the temperature within the still(T_(int)) is plotted on the graph in the form of squares and the changein the temperature at the top of the column (T_(top)) is plotted in theform of solid circles.

As will be seen, using a process according to the invention it has beenpossible to obtain a yield of the order of 53.3%.

Although this invention has been described in the form of its preferredembodiment, the main concept underlying this invention, which is aprocess for the distillation of decanted oil with the object ofincreasing the efficiency of the production of pitches having excellentphysical and chemical properties rendering them suitable for use invarious industrial applications, including the manufacture of carbonfibres, retains its innovative nature, while those skilled in the artmay envisage and make variations, modifications, changes, adaptationsand the like which are necessary and compatible with the method ofworking in question without however going beyond the spirit and scope ofthis invention which is represented by the following claims.

1. Process for the distillation of decanted oils for the production ofpetroleum pitches, characterised in that it comprises the introductionof a stage of condensation of the light and volatile compounds derivingfrom the distillation of decanted oil which makes these compoundssuitable for recycling to the hot zone of a modified still (1), reachinga maximum temperature within the range between 430° C. and 435° C. for atotal distillation time in the range between 7 and 10 hours. 2.Distillation process according to claim 1, characterised in that thesaid still (1) is equipped with a reflux system comprising a coil (2),preferably of copper, surrounding the distillation column (3) and inthat a cooling fluid, which may be selected from water, compressed airand any other fluid capable of meeting the heat-exchange capabilityrequirements, flows within the coil (2).
 3. Process of distillationaccording to claim 1, characterised in that the light compoundsrecycled: react with the raw material; act as a solvent phase of lowerviscosity than the reaction system; inhibit the rapid condensation ofcomponents of greater molecular weight; assist the breaking of chains incomponents of greater molecular weight which delay the formation ofcompounds which are insoluble in quinoline; and do away with the need tointroduce external solvent compounds into the system.
 4. Distillationprocess according to claim 1, characterised in that the stage ofcondensing the light compounds is performed by attaching a coil (2)containing a cooling fluid which circulates within it around adistillation column (3) of a still (1).